Field of the Invention
The present invention relates to an image forming apparatus that includes a developing device having a plurality of developer bearing members and forms an image according to an electrophotographic recording method or the like.
Description of the Related Art
Conventionally, an image forming apparatus such as a copying machine, a laser beam printer, a facsimile, or a printing apparatus which uses an electrophotographic system uniformly charges the surface of an image bearing member and performs image exposure with the aid of a semiconductor laser or an LED to thereby form an electrostatic latent image on the image bearing member. The electrostatic latent image is visualized as a developer image by a developing device. After that, this visible image is transferred to a transfer member, and the transferred visible image (developer image) is fixed to the transfer member by a fixing device and is output.
In recent years, the demand for improving the printing speed and the image quality of an image forming apparatus has increased, and a developing device provided in such a high-speed image forming apparatus that prints images at a high speed includes a plurality of developer bearing members that bears a developer.
Specifically, a developing device having a plurality of developer bearing members is proposed as disclosed in Japanese Patent Laid-Open No. 2000-305352 and Japanese Patent Laid-Open No. 2004-29569. The developing device disclosed in Japanese Patent Laid-Open No. 2000-305352 uses a magnetic mono-component developer as a developer.
More specifically, in the developing device, as illustrated in FIG. 1, a first developing sleeve 41a and a second developing sleeve 41b are disposed closely to each other in an opening of the developing device 4 having a developer stored therein, facing a photosensitive member 2.
The photosensitive member 2 rotates in the direction indicated by an arrow in the drawing and the first developing sleeve 41a and the second developing sleeve 41b rotates in the direction indicated by an arrow in the drawing. That is, when the first developing sleeve 41a is at a position near the photosensitive member 2, the moving direction of the photosensitive member 2 is the same as the moving direction of the first developing sleeve 41a. Moreover, when the second developing sleeve 41b is at a position near the photosensitive member 2, the moving direction of the photosensitive member 2 is the same as the moving direction of the second developing sleeve 41b. In a portion (hereinafter referred to as a “SS portion”) where the first developing sleeve 41a and the second developing sleeve 41b face each other at a close distance, the moving direction of the first developing sleeve 41a is opposite to the moving direction of the second developing sleeve 41b. 
The developer in the developing device 4 is conveyed to the vicinity of the second developing sleeve 41b by agitating and conveying members 44 and 45 and is further conveyed to the vicinity of the SS portion with rotation of the second developing sleeve 41b in the direction indicated by the arrow in the drawing. Here, when the developer passes through the SS portion, the thickness thereof is regulated by the first developing sleeve 41a and a developer layer is formed on the surface of the second developing sleeve 41b. Although a portion of the developer layer closest to the photosensitive member 2 is provided for development, a developer which has not been provided for development is collected again into the developing device 4.
On the other hand, a developer which has not been born on the surface of the second developing sleeve 41b among the developer conveyed up to the vicinity of the SS portion is conveyed to the vicinity of a thickness regulating member 42 with rotation of the first developing sleeve 41a in the direction indicated by the arrow in the drawing. When the developer passes through a gap (hereinafter referred to as a “SB portion”) between the thickness regulating member 42 and the first developing sleeve 41a, the thickness thereof is regulated by the thickness regulating member 42, and a developer layer is formed on the surface of the first developing sleeve 41a. Although a portion of the developer layer closest to the photosensitive member 2 is provided for development, a developer which has not been provided for development is conveyed to the SS portion in which the first developing sleeve 41a and the second developing sleeve 41b face each other at a close distance. A portion of the developer conveyed to the SS portion is collected into the developing device 4, and the remaining developer is conveyed to the second developing sleeve 41b to form a portion of the developer layer on the second developing sleeve 41b. 
In such a system that performs development using a plurality of developing sleeves, when the developer layer of the first developing sleeve 41a is formed in the SB portion of the developing device, a portion of an external additive included in the developer is separated from the developer to accumulate in the SB portion as clusters of agglomerates. Further, a portion of the clusters of agglomerates accumulating in the SB portion passes through the SB portion at a certain time to reach the SS portion. These clusters of agglomerates having moved to the SS portion remain in a certain longitudinal position near the SS portion and accumulate gradually as illustrated in FIG. 2. The clusters of agglomerates accumulating in a certain amount or more inhibit formation of a coated layer of the second developing sleeve 41b. When formation of the coated layer of the second developing sleeve 41b is inhibited, the coated layer of the second developing sleeve becomes thinner than that of a portion where formation of the coated layer is not inhibited. When image formation is performed in this state, as illustrated in FIG. 2, an image defect which becomes a white stripe occurs on a halftone image at a position identical to the position where clusters of agglomerates accumulate in the longitudinal direction.